Method and apparatus for controlling the concentration of carbon dioxide in an aircraft cabin

ABSTRACT

A control and method for maintaining the concentration of carbon dioxide in the cabin (11) of an aircraft at a desired level. A carbon dioxide sensor (40, 41) is disposed to monitor the concentration of carbon dioxide in the cabin of the aircraft. Air drawn from the cabin at locations (24, 25) where the concentration of carbon dioxide is relatively higher than at other locations (31, 32) is selectively vented overboard or recirculated in response to the concentration of carbon dioxide in the cabin. As more air is vented overboard to maintain the cabin at a desired level of carbon dioxide concentration, an air pack compensates by supplying additional pressurized fresh air to the cabin. Should the cabin pressure drop below a minimum set point limit, the control causes less air to be vented overboard, irrespective of the carbon dioxide concentration. In another embodiment, cabin pressure is controlled between predetermined limits, and so long as the pressure remains between those limits, additional pressurized fresh air is provided if the level of carbon dioxide increases beyond a predetermined limit.

TECHNICAL FIELD

This invention generally relates to ventilation systems for pressurizedenclosed spaces, and more particularly to a system for providing anaircraft ventilation system responsive to carbon dioxide concentration.

BACKGROUND ART

Passenger aircraft designed to fly at altitudes in excess of 10,000 feetare generally required to have sealed cabins supplied with pressurized,temperature conditioned air. Although a substantial portion of thesupply air is drawn from the cabin and recirculated, at least some freshair from outside the cabin must be pressurized and mixed with therecirculated air to make up for leakage and to provide adequateventilation for the cabin. The apparatus responsible for pressurizingand temperature conditioning fresh supply air is referred to in theaircraft industry as an "air pack."

An air pack uses a compressor for pressurizing the relatively lowdensity air at high altitudes. Typically, the compressor is powered byjet engine bleed air, using an air turbine drive. Since use of enginebleed air for this purpose reduces the operating efficiency of anaircraft, it is desirable to minimize the volume of low density freshair that the air pack must pressurize or supply to the cabin space.However, it should be apparent that at least some of the cabin air mustbe vented overboard to avoid the build up of carbon dioxide and othercontaminants that result principally from the metabolic activity of thepassengers in the aircraft.

Exacerbating the ventilation problem is the contamination produced bycigarette smoke. Although cigarette smoking passengers are generallyseated in a limited "smoking area" of the aircraft, there is a growingconceern about the rights of non-smoking airline passengers to breatheair free of cigarette smoke and its associated carcinogeniccontaminants. A conventional aircraft ventilation system may be set upto exhaust or vent a fixed percentage of the total return air overboard;however, such systems do not provide means to insure that adequate freshair is being supplied to properly ventilate the cabin, nor means toavoid venting too much air drawn from the cabin. If too little air isvented overboard and thus too little fresh air is supplied as makeup,the cabin air will eventually become stale and unhealthy. Conversely, iftoo much air is vented, the system must use excessive engine bleed airto run the air pack compressor, in order to supply pressurized fresh airfor maintenance of the cabin pressure within the desired limits.

The prior art ventilation systems used on aircraft have generally onlypermitted manual control of the amount of air vented from the aircraft.Such systems have not provided means for automatically determiningwhether more or less cabin air should be vented overboard rather thanbeing recirculated. While maintenance of a comfortable and healthyenvironment in the aircraft cabin undoubtedly has a higher priority, itis also important to minimize the operating costs of an aircraft. Priorart ventilation systems for aircraft have simply relied upon thesubjective evaluation of the flight crew to control the amount of freshair supplied to the cabin and thus often may fail either to provideproper ventilation or to operate at optimum efficiency.

SUMMARY OF THE INVENTION

The present invention is used in a system that maintains a pressurizedenclosed space (e.g., an aircraft cabin) at a desired air pressure.Included in the system are means for recirculating back into theenclosed space air drawn from a plurality of locations, and means forsupplying pressurized air that is a mixture of fresh air andrecirculated air to the enclosed space to maintain it at a desiredpressure. The invention is an apparatus and method for selectivelyrecirculating or venting air drawn from one or more locations in theenclosed space where the air is relatively higher in carbon dioxideconcentration than air drawn from one or more other locations.

The apparatus includes means for sensing carbon dioxide and producing asignal indicative of the concentration thereof in the enclosed space.Connected to receive the signal and responsive to it are, in oneembodiment, means for controlling the volume of air drawn from the oneor more locations where the air is relatively higher in carbon dioxideconcentration, that is vented from the space rather than recirculated,as a function of the concentration of carbon dioxide in the space.

As more air from the one or more locations having a relatively higherconcentration of carbon dioxide is vented, the means for supplyingpressurized air respond by supplying relatively more fresh pressurizedair to maintain the enclosed space at a desired pressure. However,should the pressure in the space fall below a preset level, the meansfor venting air that is relatively higher in concentration of carbondioxide are operative to vent less air from the space, irrespective ofthe carbon dioxide concentration.

In another embodiment, the volume of pressurized fresh air supplied tothe space is increased if the concentration of carbon dioxide exceeds apredetermined level. However, pressure control means give priority tothe maintenance of cabin pressure, irrespective of the concentration ofcarbon dioxide.

It will be apparent that the present invention operates to automaticallymaintain the enclosed space at a desired air pressure while at the sametime maintaining the concentration of carbon dioxide in the space at anacceptable level. The present invention accomplishes this object byventing only air that is relatively higher in carbon dioxide, whilerecirculating air that is lower in carbon dioxide concentration. Thusthe environmental quality of the air in the enclosed space is maintainedat a desirable level, but only the minimum air necessary to accomplishthis object is vented from the space.

As applied to the ventilation system of an aircraft, the presentinvention provides the above-noted benefit, yet improves overall systemoperating efficiency. Unnecessary use of engine bleed air to power theair pack compressor is reduced, without degradation of cabin airquality.

These and other advantages of the present invention will become apparentfrom the attached drawings and the description of the preferredembodiments that follows hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway perspective view of an aircraft cabin showingschematically a ventilation and cabin pressurization system thatincludes the present invention.

FIG. 2 is a block diagram illustrating the control relationship betweenthe ventilation and cabin pressurization system and the presentinvention.

FIG. 3 is a flow chart illustrating an algorithm for control of cabinpressure and carbon dioxide concentration, as implemented by amicroprocessor control in a first embodiment of the present invention.

FIG. 4 is a flow chart illustrating an algorithm for control of cabinpressure and carbon dioxide concentration, as implemented by amicroprocessor control in a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a section of an aircraft fuselage is shown,generally denoted by reference numeral 10. A cabin 11 within fuselage 10includes a seating area 12 having three abreast seating for passengerson each side of a center aisle. Above seating area 12 is disposed anoverhead storage bin 13 on the left side of fuselage 10, and an overheadstorage bin 14 on the right side thereof (left and right referencing thesides of fuselage 10 relative to a passenger seated in cabin 11). Thelower portion of cabin 11 is defined by cabin floor 15.

Ventilation air is supplied to passengers in seating area 12 by means ofa supply duct 16 that extends parallel to the longitudinal axis offuselage 10 at the top of cabin 11, generally centered therein andincludes a plurality of outlets (not shown) on its lower surface throughwhich air is directed generally downward toward the passenger seatingarea 12.

The ventilation supply air is distributed to supply duct 16 throughsupply air line 19. Supply air line 19 is in fluid communication with amixing manifold 23, which is provided pressurized supply air, as will bedescribed hereinbelow.

Air is drawn from cabin 11 at a number of locations. Two of theselocations, exhaust air ducts 24 and 25, extend generally parallel to thelongitudinal axis of fuselage 10 and are disposed in a corner ofoverhead storage bins 13 and 14, generally above seating area 12.Exhaust ducts 24 and 25 are thus operative to provide a flow path forair drawn from cabin 11 through a plurality of openings (not shown) at alocation immediately adjacent and above the heads of the passengersnormally seated therein, for transmission through exhaust lines 26. Oneor more fans 27, having their suction inlet connected to lines 26, areoperative to pull air from cabin 11 through ducts 24 and 25. The outletof fan(s) 27 is connected to exhaust/return line 28. Air flowing throughline 28 is directed into a three-way valve 29 that is operative toselectively vent the air overboard outside the aircraft, or pass it onto a recirculation line 30. Valve 29 may be an electro-pneumatic type ormotor driven, and operative to provide proportional control between twopositions, i.e., to vent a portion of the air overboard that is providedthrough line 28 while recirculating a portion of the air into line 30,the proportion of air thus vented overboard or recirculated beingcontrolled by the valve position. Air that is recirculated through line30 is directed into mixing manifold 23.

Two other exhaust ducts 31 and 32 extend generally parallel to thelongitudinal axis of fuselage 10 and are disposed along the sidesthereof adjacent floor 15. Air is drawn from cabin 11 at these locationsbecause they are substantially lower than exhaust ducts 24 and 25 andfarther away from the heads of passengers seated in seating area 12. Airdrawn from exhaust ducts 31 and 32 passes respectively intorecirculation lines 33 and 34, that are connected to the suction inletof one or more fans 35, which are operative to draw air from the lowerportion of cabin 11 through ducts 31 and 32. The outlet of fan(s) 35 isconnected to a recirculation line 36 that is in fluid communication witha second inlet to mixing manifold 23.

A third inlet to mixing manifold 23 is provided with pressurized air byan air pack 37. Although represented in FIG. 1 as simply a fan, it willbe understood by those skilled in the art that air pack 37 includes acompressor for pressurizing relatively low density fresh air drawn fromoutside the aircraft at 38, and further includes temperatureconditioning means for both heating and cooling such air. The mechanismby which these functions of air pack 37 are accomplished is not shown,simply because it is so well known and understood by those skilled inthe art. Pressurized and temperature conditioned air from air pack 37 isprovided as an input to mixing manifold 23 through fresh air supply line39.

Mixing manifold 23 is thus provided air from three sources: (a) airrecirculated from exhaust ducts 24 and 25 disposed above the passengerseating area 12; (b) air recirculated from exhaust ducts 31 and 32disposed near the floor of cabin 11; and (c) fresh air from outside theaircraft, pressurized and temperature conditioned by air pack 37. Theair from these three sources is mixed in the same proportion in which itis supplied to mixing manifold 23 and is provided as supply air to cabin11, as previously explained hereinabove.

Air supplied to cabin 11 passes through a carbon dioxide sensor 40which, as shown in FIG. 1, is disposed on supply air line 19. It will beunderstood that carbon dioxide sensor 40 might also be located on otherair lines, or within supply duct 16. Disposed in any of these locations,sensor 40 is operative to sense the concentration of carbon dioxide inthe supply air.

Alternatively, a carbon dioxide sensor could be located as shown at 41,disposed on recirculation air line 33. For sensing the concentration ofcarbon dioxide in the return air, it will also be apparent that sensor41 could be located within exhaust ducts 31 or 32 or on recirculationline 34.

If a carbon dioxide sensor providing a similar function to that ofsensors 40 and 41 is located within cabin 11, it should preferably bedisposed adjacent floor 15, below seating area 12. The disposition ofthe carbon dioxide sensor within cabin 11 is important because of thenon-homogenous distribution of carbon dioxide. Carbon dioxide exhaled bypassengers in seating area 12, and cigarette smoke and othercontaminants in the cabin air tend to concentrate at the upper portionof the cabin. It is undesirable to locate the carbon dioxide sensor atthis point of higher carbon dioxide concentration, because the sensorwould not be exposed to air representative of the average concentrationin the cabin. For the same reason, it is preferable that air drawn fromthese locations of relatively higher concentration of carbon dioxide andother contaminants be selectively exhausted overboard by means ofthree-way valve 29. Air drawn from a much lower elevation in cabin 11through ducts 31 and 32 has a relatively low concentration of carbondioxide and other contaminants, and thus may be recirculated. Sensors 40and 41 (and any such sensor positioned within cabin 11 as explainedabove) are each operative to indicate a representative concentration ofcarbon dioxide in cabin 11. It will be apparent that two or more suchsensors might be used for this purpose, and further, that a weightedaverage of the signals produced by sensors disposed at differentlocations might be used as a more accurate indication of carbon dioxideconcentration in the cabin space.

Three-way valve 29 is controlled in response to the concentration ofcarbon dioxide in cabin 11 as shown in FIG. 2. As already explained,carbon dioxide sensor 40 (or 41) senses the concentration of carbondioxide in air supplied to cabin 11 (or returning therefrom). Sensor 40(or 41) produces a signal indicative of the relative concentration ofcarbon dioxide in cabin 11 that is conveyed over conductors 50 to carbondioxide meter 51. In the preferred embodiment, carbon dioxide meter 51produces an analog DC signal proportional to the concentration of carbondioxide in the cabin, having a range from 0 to 5 VDC. A sensor 40 (or41) and carbon dioxide meter 51 appropriate for use in this applicationis manufactured by Gas Tech, Inc. (Model 3600). This meter uses aninfrared absorption sensor to generate an electrical signal that variesinversely with the carbon dioxide level in a gas being sampled. Otherdevices producing an output signal that varies in proportion to CO₂concentration might also be used for this purpose.

In a first embodiment of the present invention, the analog signalproduced by carbon dioxide meter 51 is input over conductor 52' to avalve controller 53. In response to the analog signal input on line 52',valve controller 53 produces either a pneumatic or an electrical valvecontrol signal (depending on the type valve 29 used) that is carried vialine 54 to three-way valve 29, controlling the amount of air that isvented overboard. For example, assume that the maximum concentration ofcarbon dioxide desired in cabin 11 as measured by sensor 40 is 0.043%.Should the concentration of carbon dioxide exceed that level, valvecontroller 53 produces a signal that is input over line 54 to three-wayvalve 29, causing it to vent overboard relatively more air drawn fromcabin 11 through exhaust ducts 24 and 25. Conversely, as theconcentration of carbon dioxide determined by sensor 40 falls below thedesired level, valve controller 53 causes three-way valve 29 to ventless air overboard.

If the carbon dioxide sensor is located at 41, the desirable limit maybe set at a relatively higher percentage, for example 0.1%. Valvecontroller 53 nevertheless functions in the same manner to maintain theconcentration of carbon dioxide in the cabin as measured at sensor 41(or at a location in cabin 11 adjacent floor 15) at the desired 0.1%limit.

As valve controller 53 responds to a carbon dioxide concentration levelin excess of the desired limit, causing three-way valve 29 to ventoverboard more air drawn from exhaust ducts 24 and 25, the pressure incabin 11 tends to decrease. A pressure sensor 55 schematically shown inFIG. 1 disposed adjacent the top of cabin 11 provides an input topressure controller 57 as shown in FIG. 2. Pressure controller 57responds to the air pressure in cabin 11 as measured by pressure sensor55 to cause air pack 37 to supply either more or less pressurized freshair from outside the aircraft. Since there is usually some leakage fromthe cabin 11 as represented by block 42 in FIG. 2, pressure controller57 generally requires that at least some make-up fresh air be suppliedby air pack 37, regardless of any action taken to vent more airoverboard by three-way valve 29. However, in responding to a decreasingcabin air pressure caused by three-way valve 29 venting an increasingamount of air, pressure controller 57 causes air pack 37 to supply acorresponding increased volume of pressurized fresh air to mixingmanifold 23, thereby maintaining the cabin pressure at a desired level.

In the event that cabin pressure falls below a preset minimum limit, asmight occur should a window or door seal fail, pressure controller 57produces an override signal that is input to valve controller 53 bymeans of conductor 58. This override signal causes valve controller 53to selectively vent less air overboard through valve 29, enabling airpack 37 to increase the air pressure within cabin 11 to an acceptablelevel.

The functions performed by pressure controller 57 and valve controller53 can be implemented by a conventional microprocessor control (notshown) provided the control is properly programmed with the appropriatecontrol algorithms. Since the details of typical microprocessorcontrollers are generally well known to those skilled in the art, it isnot necessary to present them herein to provide a full enablingdisclosure of the present invention. As is typical of such controls, themicroprocessor control should include an analog-to-digital conversioncapability so that analog signals from pressure sensor 55 and fromcarbon dioxide sensor 40 (41) can be converted to a digital signal thatthe microprocessor may use as input. Likewise, control of air pack 37and three-way valve 29 by the microprocessor may be implemented using aconventional digital-to-analog converter (and if pneumatic control isrequired, an electrical-to-pneumatic signal transducer). Amicroprocessor implementing the control functions of pressure controller57 and valve controller 53 would typically use a control program storedin read only memory (ROM). An algorithm for the first embodiment isrepresented by the flow chart of FIG. 3.

As shown in FIG. 3, the control program initiates at start block 60, andproceeds to block 61 wherein the microprocessor control checks todetermine if cabin pressure is below a desired setting. If the pressureis too low, the microprocessor causes the air pack to increase its flowof pressurized fresh air into cabin 11, as indicated in block 62.Thereafter (or if the cabin pressure is not too low), in block 63, themicroprocessor control checks to determine if cabin pressure exceeds anupper limit. If the cabin pressure is too high, in block 64, the controlcauses air pack 37 to decrease the flow of pressurized fresh air intomixing manifold 23. In block 65, the control determines if the carbondioxide level sensed by sensor 40 (41) is higher than a desirable limit.If the carbon dioxide level does exceed the desired limit, in block 66,the control checks to determine if the cabin pressure is less than aminimum set point. If the pressure is less than the set point, in block69, the control decreases the flow of air vented overboard by causingthree-way valve 29 to recirculate back into cabin 11 more of the airdrawn out through exhaust ducts 24 and 25. Otherwise, in block 67,three-way valve 29 is caused to increase the amount of air ventedoverboard.

Referring back to block 65, if the carbon dioxide level is not too high,the control logic proceeds to block 68 where the control checks to seeif the concentration of CO₂ is below the desired limit, and if it is,the step previously discussed in block 69 is implemented, i.e., theamount of air being vented overboard is decreased. Otherwise, asindicated in block 70, the algorithm repeats, returning to the start inblock 60.

A second embodiment of the present invention is implemented byconnecting the carbon dioxide meter 51 to pressure controller 57 by lead52, so that air pack 37 is more directly used to maintain theconcentration of carbon dioxide in cabin 11 within acceptable limits byvarying the amount of fresh pressurized air supplied to the cabin. Valvecontrol 53 continues to control three-way valve 29 to vent airoverboard, but is not directly connected to carbon dioxide meter 51, andis not directly responsive to the concentration of carbon dioxide in thecabin. Lead 52' is thus omitted in the second embodiment.

An algorithm for controlling cabin pressure and carbon dioxideconcentration in cabin 11 in accord with the second embodiment isillustrated in the flow chart of FIG. 4. Like the first embodiment, itis anticipated that valve control 53 and pressure controller 57 mightcomprise a conventional microprocessor control including the functionalA-D, D-A and memory components already described. A control program forimplementing the algorithm as stored in ROM starts at block 100 andcontinues with block 101, where the control determines if the pressurein cabin 11 is less than a predetermined minimum limit. If the pressureis too low, in block 102, the control causes valve 29 to decrease thevolume of air drawn through exhaust ducts 24 and 25 that is being ventedoverboard, and in block 103, causes air pack 37 to increase the volumeof pressurized fresh air that is supplied to cabin 11 by air pack 37.The program then recycles to start, in block 100.

However, if cabin pressure is not too low, in block 104, the algorithmchecks to determine if the pressure is greater than a predeterminedlimit, and if so, in blocks 105 and 106, respectively, increases thevolume of air exhausted overboard from the cabin by valve 29, anddecreases the volume of pressurized fresh air supplied to the cabin byair pack 37. Again, the program recycles to start, in block 100.

Once the cabin pressure is within the predetermined limits, programlogic ascertains if the concentration of carbon dioxide is withinpredetermined upper and lower limits, in blocks 107 and 109. If the CO₂level is too high, the control causes air pack 37 to increase the flowof pressurized fresh air into the cabin, in block 108, or conversely, ifthe CO₂ level is too low, to decrease the flow of pressurized fresh airinto the cabin, in block 110. Following either action, the controlrecycles to start, as it also does if the level of carbon dioxideconcentration is within limits, in block 111.

Although the net result of using the first and the second embodiments issimilar, the second embodiment is believed to provide more stablecontrol, particularly should a drastic decrease in cabin pressure occurdue to an emergency or seal failure. In both embodiments, themaintenance of cabin pressure at a desired level (or between desiredlimits) has priority over controlling carbon dioxide concentration at adesired level (or between limits).

The functions of cabin pressure control and control of three-way valve29 in response to the 0-5 VDC signal from meter 51 as described above inboth embodiments are also readily implemented using available discreteelectrical and/or mechanical components instead of the microprocessor,as will be apparent to those skilled in the art. Disclosure of thedetails of valve controller 53 and pressure controller 57 comprisingsuch components is thus unnecessary to enable the present invention tobe built and used.

Although the present invention has been disclosed with respect topreferred embodiments and modifications thereto, further modificationswill be apparent to those skilled in the art within the scope of theclaims which follow hereinbelow.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus forcontrolling the ventilation of a pressurized enclosed space,comprising:(a) a carbon dioxide sensor disposed so as to monitor carbondioxide concentration in the enclosed space; (b) a valve disposed in afluid path through which air drawn from the space is selectively ventedfrom the space or recirculated back into it; (c) air pack means forsupplying fresh pressurized air to the space; and (d) control means,connected to the carbon dioxide sensor, the pressurization means and thevalve, for controlling as a function of the concentration of carbondioxide in the space one of (i) the volume of pressurized fresh airsupplied to the space by the pressurization means, and (ii) the volumeof air vented from the space through the valve.
 2. The apparatus ofclaim 1 wherein the control means controls the proportion of the airdrawn from the space that is vented through the vale relative to thatwhich is recirculated back into the space as a function of theconcentration of carbon dioxide in the space.
 3. The apparatus of claim1 wherein air is drawn from the enclosed space at a first location and asecond location, the concentration of carbon dioxide being relativelygreater at the first location than at the second.
 4. The apparatus ofclaim 3 further comprising a mixing manifold, wherein the air drawn fromthe space at the second location is mixed in the mixing manifold withair which has passed through the valve after being drawn from the spaceat the first location, the mixed air being recirculated back into thespace.
 5. The apparatus of claim 4 wherein the fresh pressurized air ismixed in the mixing manifold with the air drawn from the space at thefirst and second locations and the mixed air is recirculated back intothe space.
 6. The apparatus of claim 5 further comprising a pressuresensor disposed in the enclosed space, wherein the control meanscomprise pressure control means connected to and responsive to thepressure sensor, for controlling the pressure in the enclosed space at apredetermined level by modulating the flow of the pressurized fresh airinto the mixing manifold.
 7. The apparatus of claim 6 wherein if thepressure in the enclosed space falls below a preset level, the pressurecontrol means are further operative to reduce the volume of air ventedfrom the space by the valve, irrespective of the concentration of carbondioxide in the enclosed space.
 8. The apparatus of claim 1 wherein thecontrol means are operative to cause the air pack means to (a) increasethe volume of pressurized fresh air supplied to the space if theconcentration of carbon dioxide exceeds a first predetermined level, and(b) to decrease the volume of pressurized fresh air supplied to theenclosed space if the concentration of carbon dioxide is less than asecond predetermined level.
 9. The apparatus of claim 8 furthercomprising a pressure sensor disposed in the enclosed space, wherein thecontrol means comprise pressure control means connected to andresponsive to the pressure sensor, for controlling the volume ofpressurized fresh air supplied to the space in order to maintain thepressure in the space between predetermined limits, irrespective of theconcentration of carbon dioxide in the space.
 10. In a system formaintaining a pressurized enclosed space at a desired air pressure,including: means for recirculating back into the enclosed space airdrawn from the space at a plurality of locations, the air drawn from atleast one or more locations being relatively higher in carbon dioxideconcentration than air drawn from one or more other locations; and meansfor supplying pressurized air that is a mixture of fresh air andrecirculated air to the enclosed space to maintain it at a desiredpressure, apparatus for controlling the ventilation of the space,comprising:(a) means for sensing carbon dioxide and for producing asignal indicative of the concentration thereof in the enclosed space;(b) means connected to receive the signal and responsive thereto, forcausing one of (i) a relatively greater volume of air tobe drawn fromthe one or more locations where the air is relatively higher in carbondioxide concentration and vented from the space rather than recirculatedand mixed into the pressurized supply air, and (ii) the means forsupplying pressurized air to supply relatively more fresh air to thespace, if the concentration of carbon dioxide indicated by the signalexceeds a predetermined level.
 11. The apparatus of claim 10 wherein themeans for sensing carbon dioxide include a sensor disposed in a flowpath for the mixture of fresh air and recirculated air supplied to theenclosed space.
 12. The apparatus of claim 10 wherein the means forsensing carbon dioxide include a sensor disposed adjacent to one of thelocations where the air is relatively lower in carbon dioxideconcentration.
 13. The apparatus of claim 10 wherein the means connectedto receive the signal indicative of carbon dioxide concentration arefurther operative to cause less air to be vented from the enclosed spaceif the pressure in the enclosed space falls below a preset level. 14.The apparatus of claim 10 wherein the air from the one or more locationsthat is relatively higher in carbon dioxide concentration and which isrecirculated back into the space is mixed with the air from the one ormore other locations and with fresh air prior to being supplied to theenclosed space.
 15. A method for maintaining an enclosed space at adesired pressure while controlling the concentration of carbon dioxidein a pressurized airstream supplied to the space, the airstream suppliedthereto being a mixture of fresh air and of recirculated air drawn froma first and a second location in the space characterized by a relativelyhigher concentration of carbon dioxide in the first location than in thesecond, comprising the steps of:(a) monitoring the concentration ofcarbon dioxide in the enclosed space; (b) monitoring the pressure in theenclosed space; (c) in response to the monitored pressure, increasingthe proportion of fresh air in the airstream supplied to the spacerelative to the recirculated air, to make up for the air vented from theenclosed space; and (d) if the monitored concentration of carbon dioxidein the space exceeds a predetermined level, causing one of (i) more ofthe air drawn from the first location to be vented outside, and (ii) anincrease in the proportion of fresh air in the airstream supplied to thespace.
 16. The method of claim 15 wherein all of the air drawn from thesecond location is recirculated back into the space and not vented. 17.The method of claim 15 wherein the concentration of carbon dioxide ismonitored by sensing the concentration of carbon dioxide in thepressurized airstream supplied to the space.
 18. The method of claim 15further comprising the step of reducing the volume of air that is ventedfrom the space, if the monitored pressure in the space falls below apreset level, irrespective of the concentration of carbon dioxide in thespace.
 19. The method of claim 15 wherein relatively more air drawn fromthe first location is vented as the concentration of carbon dioxidewithin the enclosed space increases.
 20. The method of claim 15 whereinthe enclosed space is an aircraft cabin and the first location isdisposed above a seating area in the cabin, while the second location isdisposed adjacent a floor of the cabin.
 21. The method of claim 15wherein a source of the carbon dioxide comprises a metabolic activityoccurring within the enclosed space.
 22. The method of claim 15 whereina source of the carbon dioxide comprises cigarette smoke.